diff --git a/.changeset/entity-tab-not-found.md b/.changeset/entity-tab-not-found.md
new file mode 100644
index 0000000000..074c0a9e42
--- /dev/null
+++ b/.changeset/entity-tab-not-found.md
@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-catalog': patch
+---
+
+Fixed an issue where navigating to an unknown sub-path on an entity page (for example `/catalog/default/component/foo/blob`) would silently render the first available route. Unknown paths now show the standard not-found page instead.
diff --git a/packages/core-compat-api/src/convertLegacyApp.test.tsx b/packages/core-compat-api/src/convertLegacyApp.test.tsx
index 7094eb798c..9c6728321d 100644
--- a/packages/core-compat-api/src/convertLegacyApp.test.tsx
+++ b/packages/core-compat-api/src/convertLegacyApp.test.tsx
@@ -317,9 +317,10 @@ describe('convertLegacyApp', () => {
       features: [catalogOverride, ...converted],
       initialRouteEntries: ['/catalog/default/other/x/bar'],
     });
+    // /bar does not exist on the "other" entity layout, expect the not-found page.
     await expect(
-      renderBarOther.findByText('other overview content', {}, findOptions),
-    ).resolves.toBeInTheDocument(); // /bar does not exist, fall back to rendering overview
+      renderBarOther.findByTestId('error', {}, findOptions),
+    ).resolves.toBeInTheDocument();
     renderBarOther.unmount();
   });
 });
diff --git a/plugins/catalog/src/alpha/components/EntityTabs/EntityTabs.test.tsx b/plugins/catalog/src/alpha/components/EntityTabs/EntityTabs.test.tsx
index a85d795cd8..3145006fa8 100644
--- a/plugins/catalog/src/alpha/components/EntityTabs/EntityTabs.test.tsx
+++ b/plugins/catalog/src/alpha/components/EntityTabs/EntityTabs.test.tsx
@@ -15,7 +15,7 @@
  */
 
 import { screen } from '@testing-library/react';
-import { useSelectedSubRoute } from './EntityTabs';
+import { EntityTabs, useSelectedSubRoute } from './EntityTabs';
 import { EntityTabsList } from './EntityTabsList';
 import { MemoryRouter, Route, Routes } from 'react-router-dom';
 import { render } from '@testing-library/react';
@@ -139,7 +139,7 @@ describe('EntityTabs', () => {
   ];
 
   describe('useSelectedSubRoute', () => {
-    it('should render the first route at root path', () => {
+    it('should not match when no route is registered for the root path', () => {
       render(
         <MemoryRouter initialEntries={['/']}>
           <Routes>
@@ -151,10 +151,74 @@ describe('EntityTabs', () => {
         </MemoryRouter>,
       );
 
+      expect(screen.getByTestId('selected-index')).toHaveTextContent('-1');
+      expect(screen.getByTestId('element-container')).toBeEmptyDOMElement();
+    });
+
+    it('should match the index route when path is "/"', () => {
+      const subRoutesWithIndex = [
+        {
+          group: 'default',
+          path: '/',
+          title: 'Overview',
+          children: <div>Overview Content</div>,
+        },
+        {
+          group: 'default',
+          path: '/details',
+          title: 'Details',
+          children: <div>Details Content</div>,
+        },
+      ];
+
+      render(
+        <MemoryRouter initialEntries={['/']}>
+          <Routes>
+            <Route
+              path="/*"
+              element={<TestSubRouteHook subRoutes={subRoutesWithIndex} />}
+            />
+          </Routes>
+        </MemoryRouter>,
+      );
+
       expect(screen.getByTestId('selected-index')).toHaveTextContent('0');
       expect(screen.getByTestId('selected-route-title')).toHaveTextContent(
         'Overview',
       );
+      expect(screen.getByText('Overview Content')).toBeInTheDocument();
+    });
+
+    it('should not let the index route swallow unknown sub-paths', () => {
+      const subRoutesWithIndex = [
+        {
+          group: 'default',
+          path: '/',
+          title: 'Overview',
+          children: <div>Overview Content</div>,
+        },
+        {
+          group: 'default',
+          path: '/details',
+          title: 'Details',
+          children: <div>Details Content</div>,
+        },
+      ];
+
+      render(
+        <MemoryRouter initialEntries={['/blob']}>
+          <Routes>
+            <Route
+              path="/*"
+              element={<TestSubRouteHook subRoutes={subRoutesWithIndex} />}
+            />
+          </Routes>
+        </MemoryRouter>,
+      );
+
+      expect(screen.getByTestId('selected-index')).toHaveTextContent('-1');
+      expect(screen.queryByText('Overview Content')).not.toBeInTheDocument();
+      expect(screen.queryByText('Details Content')).not.toBeInTheDocument();
     });
 
     it('should render a route at non-root path', () => {
@@ -276,7 +340,7 @@ describe('EntityTabs', () => {
       );
     });
 
-    it('should fall back to first route for unknown paths', () => {
+    it('should not match unknown paths', () => {
       render(
         <MemoryRouter initialEntries={['/unknown-path']}>
           <Routes>
@@ -288,10 +352,135 @@ describe('EntityTabs', () => {
         </MemoryRouter>,
       );
 
-      expect(screen.getByTestId('selected-index')).toHaveTextContent('0');
-      expect(screen.getByTestId('selected-route-title')).toHaveTextContent(
-        'Overview',
+      expect(screen.getByTestId('selected-index')).toHaveTextContent('-1');
+      expect(screen.getByTestId('element-container')).toBeEmptyDOMElement();
+    });
+
+    it('should accept paths that already include an explicit wildcard', () => {
+      const wildcardSubRoutes = [
+        {
+          group: 'default',
+          path: '/',
+          title: 'Overview',
+          children: <div>Overview Content</div>,
+        },
+        {
+          group: 'default',
+          path: '/docs/*',
+          title: 'Docs',
+          children: <div>Docs Content</div>,
+        },
+      ];
+
+      render(
+        <MemoryRouter initialEntries={['/docs/api/v1']}>
+          <Routes>
+            <Route
+              path="/*"
+              element={<TestSubRouteHook subRoutes={wildcardSubRoutes} />}
+            />
+          </Routes>
+        </MemoryRouter>,
       );
+
+      expect(screen.getByTestId('selected-index')).toHaveTextContent('1');
+      expect(screen.getByTestId('selected-route-title')).toHaveTextContent(
+        'Docs',
+      );
+      expect(screen.getByText('Docs Content')).toBeInTheDocument();
+    });
+  });
+
+  describe('rendering', () => {
+    const tabRoutes = [
+      {
+        group: 'overview',
+        path: '/',
+        title: 'Overview',
+        children: <div>Overview Content</div>,
+      },
+      {
+        group: 'overview',
+        path: '/details',
+        title: 'Details',
+        children: <div>Details Content</div>,
+      },
+    ];
+
+    it('renders the matched route content and no not-found page', async () => {
+      await renderInTestApp(
+        <Routes>
+          <Route
+            path="/*"
+            element={
+              <EntityTabs
+                routes={tabRoutes}
+                groupDefinitions={{ overview: { title: 'Overview' } }}
+              />
+            }
+          />
+        </Routes>,
+        { initialRouteEntries: ['/details'] },
+      );
+
+      expect(await screen.findByText('Details Content')).toBeInTheDocument();
+      expect(screen.queryByTestId('error')).toBeNull();
+    });
+
+    it('renders the not-found page for unknown sub-paths', async () => {
+      await renderInTestApp(
+        <Routes>
+          <Route
+            path="/*"
+            element={
+              <EntityTabs
+                routes={tabRoutes}
+                groupDefinitions={{ overview: { title: 'Overview' } }}
+              />
+            }
+          />
+        </Routes>,
+        { initialRouteEntries: ['/blob'] },
+      );
+
+      expect(await screen.findByTestId('error')).toBeInTheDocument();
+      expect(screen.queryByText('Overview Content')).not.toBeInTheDocument();
+      expect(screen.queryByText('Details Content')).not.toBeInTheDocument();
+    });
+
+    it('still routes nested sub-paths to the matching tab content', async () => {
+      const nestedRoutes = [
+        {
+          group: 'overview',
+          path: '/',
+          title: 'Overview',
+          children: <div>Overview Content</div>,
+        },
+        {
+          group: 'overview',
+          path: '/docs',
+          title: 'Docs',
+          children: <div>Docs Content</div>,
+        },
+      ];
+
+      await renderInTestApp(
+        <Routes>
+          <Route
+            path="/*"
+            element={
+              <EntityTabs
+                routes={nestedRoutes}
+                groupDefinitions={{ overview: { title: 'Overview' } }}
+              />
+            }
+          />
+        </Routes>,
+        { initialRouteEntries: ['/docs/api/v1'] },
+      );
+
+      expect(await screen.findByText('Docs Content')).toBeInTheDocument();
+      expect(screen.queryByTestId('error')).toBeNull();
     });
   });
 });
diff --git a/plugins/catalog/src/alpha/components/EntityTabs/EntityTabs.tsx b/plugins/catalog/src/alpha/components/EntityTabs/EntityTabs.tsx
index 102df76042..f9deb0dec8 100644
--- a/plugins/catalog/src/alpha/components/EntityTabs/EntityTabs.tsx
+++ b/plugins/catalog/src/alpha/components/EntityTabs/EntityTabs.tsx
@@ -16,6 +16,7 @@
 import { ReactElement, useMemo } from 'react';
 import { Helmet } from 'react-helmet';
 import { matchRoutes, useParams, useRoutes } from 'react-router-dom';
+import { NotFoundErrorPage } from '@backstage/frontend-plugin-api';
 import { EntityTabsPanel } from './EntityTabsPanel';
 import { EntityTabsList } from './EntityTabsList';
 import { EntityContentGroupDefinitions } from '@backstage/plugin-catalog-react/alpha';
@@ -28,6 +29,22 @@ type SubRoute = {
   children: JSX.Element;
 };
 
+// Normalize a route path so it can be matched correctly:
+//   - strip leading slashes
+//   - if the path already ends with a `*`, keep it as-is so explicit wildcards
+//     like `/*` or `/foo/*` aren't double-suffixed into `*/*` / `foo/*/*`
+//   - otherwise strip trailing slashes and append `/*` for nested matching;
+//     a bare `/` collapses to the empty string so it acts as an index route
+//     rather than a wildcard that would swallow every sub-path
+function normalizeRoutePath(path: string): string {
+  const withoutLeading = path.replace(/^\/+/, '');
+  if (withoutLeading.endsWith('*')) {
+    return withoutLeading;
+  }
+  const trimmed = withoutLeading.replace(/\/+$/, '');
+  return trimmed ? `${trimmed}/*` : '';
+}
+
 export function useSelectedSubRoute(subRoutes: SubRoute[]): {
   index: number;
   route?: SubRoute;
@@ -35,37 +52,32 @@ export function useSelectedSubRoute(subRoutes: SubRoute[]): {
 } {
   const params = useParams();
 
-  const routes = subRoutes.map(({ path, children }) => ({
-    caseSensitive: false,
-    path: `${path}/*`,
-    element: children,
-  }));
-
-  // TODO: remove once react-router updated
-  const sortedRoutes = routes.sort((a, b) =>
-    // remove "/*" symbols from path end before comparing
-    b.path.replace(/\/\*$/, '').localeCompare(a.path.replace(/\/\*$/, '')),
+  const routes = useMemo(
+    () =>
+      subRoutes.map(({ path, children }) => ({
+        caseSensitive: false,
+        path: normalizeRoutePath(path),
+        element: children,
+      })),
+    [subRoutes],
   );
 
-  const element = useRoutes(sortedRoutes) ?? subRoutes[0]?.children;
+  const element = useRoutes(routes) ?? undefined;
 
-  // TODO(Rugvip): Once we only support v6 stable we can always prefix
-  // This avoids having a double / prefix for react-router v6 beta, which in turn breaks
-  // the tab highlighting when using relative paths for the tabs.
   let currentRoute = params['*'] ?? '';
   if (!currentRoute.startsWith('/')) {
     currentRoute = `/${currentRoute}`;
   }
 
-  const [matchedRoute] = matchRoutes(sortedRoutes, currentRoute) ?? [];
+  const [matchedRoute] = matchRoutes(routes, currentRoute) ?? [];
   const foundIndex = matchedRoute
-    ? subRoutes.findIndex(t => `${t.path}/*` === matchedRoute.route.path)
-    : 0;
+    ? routes.findIndex(r => r.path === matchedRoute.route.path)
+    : -1;
 
   return {
-    index: foundIndex === -1 ? 0 : foundIndex,
+    index: foundIndex,
     element,
-    route: subRoutes[foundIndex] ?? subRoutes[0],
+    route: subRoutes[foundIndex],
   };
 }
 
@@ -83,8 +95,8 @@ export function EntityTabs(props: EntityTabsProps) {
 
   const tabs = useMemo(
     () =>
-      routes.map(t => {
-        const { path, title, group, icon } = t;
+      routes.map(r => {
+        const { path, title, group, icon } = r;
         let to = path;
         // Remove trailing /*
         to = to.replace(/\/\*$/, '');
@@ -112,7 +124,7 @@ export function EntityTabs(props: EntityTabsProps) {
       />
       <EntityTabsPanel>
         <Helmet title={route?.title} />
-        {element}
+        {element ?? <NotFoundErrorPage />}
       </EntityTabsPanel>
     </>
   );
diff --git a/plugins/catalog/src/alpha/components/EntityTabs/EntityTabsList.tsx b/plugins/catalog/src/alpha/components/EntityTabs/EntityTabsList.tsx
index 9abbfb69d9..4885fac1d7 100644
--- a/plugins/catalog/src/alpha/components/EntityTabs/EntityTabsList.tsx
+++ b/plugins/catalog/src/alpha/components/EntityTabs/EntityTabsList.tsx
@@ -170,7 +170,7 @@ export function EntityTabsList(props: EntityTabsListProps) {
     return sorted;
   }, [items, groupDefinitions, aliasToGroup, defaultContentOrder]);
 
-  const selectedItem = items[selectedIndex];
+  const selectedItem = selectedIndex >= 0 ? items[selectedIndex] : undefined;
   const selectedGroup = resolveGroupId(
     selectedItem?.group,
     groupDefinitions,
@@ -185,7 +185,7 @@ export function EntityTabsList(props: EntityTabsListProps) {
         variant="scrollable"
         scrollButtons="auto"
         aria-label={t('entityTabs.tabsAriaLabel')}
-        value={selectedGroup ?? selectedItem?.id}
+        value={selectedGroup ?? selectedItem?.id ?? false}
       >
         {groups.map(([id, tabGroup]) => (
           <EntityTabsGroup